Evidence for significance of other-than-normal diffusion transport in soil gas exchange — a discussion

Abstract

Elemental Hg (Hg0) evolution from soils can be an important process and needs to be measured in more ecosystems. The diffusion model for soil gaseous efflux has been applied to modeling the fluxes of several gases in soils and deserves testing with regard to Hg0. As an initial test of this model, we examined soil gaseous Hg0 and CO2 concentrations at two depths (20 and 40 cm) over the course of a controlled environment study conducted in the EcoCELLs at the Desert Research Institute in Reno, Nevada. We also compared small, spatially distributed gas wells against the more commonly used large gas wells. In this study, two EcoCELLs were first watered (June 2000) and then planted (July 2000) with trembling aspen (Populus tremuloides). Following that, trees were harvested (October 2000) and one EcoCELL (EcoCELL 2) was replanted with aspen (25 April 2001). During most of the experiment, there was a strong vertical gradient of CO2 (increasing with depth, as is typical of a diffusion-driven process), but no vertical gradient of soil gaseous Hg0. Strong diel variations in soil gas Hg0 concentration were noted, whereas diel variations in CO2 were small and not statistically significant. Initial watering and planting caused increases in both soil gas CO2 and Hg0. Replanting in EcoCELL 2 caused a statistically significant increase in soil gas CO2 but not Hg0. Calculated Hg0 effluxes using the diffusion model produced values two orders of magnitude lower than those measured using field chambers placed directly on the soil or whole-cell fluxes. Neither soil gas Hg0 concentrations nor calculated fluxes were correlated with measured Hg0 efflux from soil or from whole EcoCELLs. We conclude that (1) soil gas Hg0 flux is not diffusion-driven and thus soil gas Hg0 concentrations cannot be used to calculated soil Hg0 efflux; (2) soil gas Hg0 concentrations are increased by watering dry soil, probably because of displacement/desorption processes; (3) soil gas Hg0 concentrations were unaffected by plants, suggesting that roots and rhizosphere processes are unimportant in controlling Hg0 evasion from the soil surface. We recommend the use of the small wells in all future studies because they are much easier to install and provide more resolution of spatial and temporal patterns in soil gaseous Hg0.